MIC502_06 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – Fan Management IC

Micrel, Inc.
incorporated into the design then the operating voltage
(V_DD = 5V vs. V_DD = 12V) becomes a concern. The FanC
signal is derived from a 12V supply and is specified to
swing at least to 10.5V. A minimum implementation of
the FanC signal would provide the capability of asserting
full-speed operation of the fan; this is the case when
10.5V ≤ FanC ≤ 12V. This FanC signal can be applied
directly to the V_T2 input of the MIC502, but only when its
V_DD is 12V. If this signal is required when the MIC502
V_DD = 5V a resistor divider is necessary to reduce this
input voltage so it does not exceed the MIC502 V_DD
voltage. A good number is 4V (80%V_DD).
Because of input leakage considerations, the impedance
of the resistive divider should be kept at ≤ 100kΩ. A
series resistor of 120kΩ driven by the FanC signal and a
100kΩ shunt resistor to ground make a good divider for
driving the V_T2 input.
Transistor and Base-Drive Resistor Selection
The OUT motor-drive output, pin 7, is intended for
driving a medium-power device, such as an NPN
transistor. A rather ubiquitous transistor, the 2N2222A, is
capable of switching up to about 400mA. It is also
available as the PN2222A in a plastic TO-92 package.
Since 400mA is about the maximum current for most
popular computer power supply fans (with many drawing
substantially less current) and since the MIC502
provides a minimum of 10mA output current, the
PN2222A, with its minimum β of 40, is the chosen motor-
drive transistor.
The design consists solely of choosing the value R_BASE in
Figures 7 and 8. To minimize on-chip power dissipation
in the MIC502, the value of R_BASE should be determined
by the power supply voltage. The Electrical
Characteristics table specifies a minimum output current
of 10mA. However, different output voltage drops (V_DD –
V_OUT) exist for 5V vs.12V operation. The value R_BASE
should be as high as possible for a given required
transistor base-drive current in order to reduce on-chip
power dissipation.
Referring to the “Typical Application” and to the
“Electrical Characteristics” table, the value for R_BASE is
calculated as follows. For V_DD = 5V systems, I_OH of OUT
(pin 7) is guaranteed to be a minimum of 10mA with a
V_OH of 2.4V.
R_BASE then equals (2.4V – V_BE) ÷ 10mA = 170Ω.
For V_DD = 12V systems, R_BASE = (3.4 – 0.7) ÷ 0.01 =
250Ω.
MIC502
Overtemperature Fault Output
The /OTF output, pin 6, is an open-collector NPN output.
It is compatible with CMOS and TTL logic and is
intended for alerting a system about an overtemperature
condition or triggering a power supply crowbar circuit. If
V_DD for the MIC502 is 5V the output should not be pulled
to a higher voltage. This output can sink up to 2mA and
remain compatible with the TTL logic-low level.
Timing Capacitors vs. PWM Frequency
The recommended C_F (see first page) is 0.1µF for
operation at a PWM frequency of 30Hz. This frequency
is factory trimmed within ±3Hz using a 0.1% accurate
capacitor. If it is desired to operate at a different
frequency, the new value for C_F is calculated as follows:
C = ³ , where C is in µF and f is in Hz
f
The composition, voltage rating, ESR, etc., parameters
of the capacitor are not critical. However, if tight control
of frequency vs. temperature is an issue, the
temperature coefficient may become a consideration.
Keystone Thermonics
RL2010-54.1K-138-D1
or similar
T1
R1
100k
R3 R2
56k 33k
R4
56k
C_F
0.1µF
NLX FanC
Signal Input
5V
12V
MIC502
1 _{VT1 VDD} 8
2 _{CF OUT} 7
3 _{VSLP OTF} 6
4 _{GND VT2} 5
Yate Loon
YD80SM-12
47k
or similar fan
R_{B A S E}
Q1
180
Overtemperature
Fault Output
120k 100k
Figure 7. Typical 5V V_DD Application Circuit
Keystone Thermonics
RL2010-54.1K-138-D1
or similar
12V 5V
T1
R1
100k
R3 R2
56k 33k
R4
56k
C_F
0.1µF
MIC502
1 _{VT1 VDD} 8
2 _{CF OUT} 7
3 _{VSLP OTF} 6
4 _{GND VT2} 5
47k
R_{B A S E}
280
Yate Loon
YD80SM-12
or similar fan
Q1
Overtemperature
Fault Output
4.7k
NLX FanC
Signal Input
Figure 8. Typical 12V V_DD Application Circuit
November 2006
13
M9999-112206